The present invention relates to an evaluation method for an inductive displacement sensor, and more particularly, to a method of determining a temperature-compensated measured value for a calibration of a displacement sensor.
An evaluation method of this type is described, for example, in DE-OS 43 18 263, the full contents of which are made the object of the present patent, and which discloses that the temperature (TIST) is to be measured at the point of installation of the displacement sensor and the measured value is to be converted in an electronic evaluation system to a normal temperature T0, e.g. 20xc2x0 C., in order to compensate for the temperature dependency of the measured value of an inductive displacement sensor, according to the known formula:       S    compensated    =            S      noncompensated              1      +              m        ⁢                  (                                    T              IST                        -                          T              0                                )                    
where the magnitudes are defined as follows:
Scompensated=temperature-compensated measured value
Snoncompensated=non-temperature-compensated measured value
m=temperature coefficient of the displacement sensor (slope of the temperature curve)
TIST=sensor temperature
T0=normal temperature
If the fact that the temperature coefficient m depends on the armature position s is additionally taken into account, the temperature coefficient m can be calculated by means of a second function which reflects the anchor position dependency and which uses a term independent of temperature as an input variable. This approach requires iterative steps.
A displacement sensor which is provided for a clutch has the task of detecting the position of the clutch. Since the measured displacement values vary significantly with the temperature, such variations attributable to temperature must be compensated for. For such purpose, the compensation method of the type described above is suitable in principle.
Clutch sensors present a particular challenge in that the spread of the measured displacement values due to the temperature fluctuations is in the same order of magnitude as the unavoidable displacement tolerances which occur when the sensor is installed. Consequently, these displacement tolerances cannot be eliminated by means of the temperature compensation method mentioned above.
It is therefore the object of the present invention to expand the sensor evaluation method of the patent application mentioned initially such that it is also suitable for application to a clutch sensor.
In accordance with this and other objects of the invention, there is provided a method of determining a temperature-compensated measured value for calibration of a displacement sensor, designed as an inductive displacement sensor, in accordance with which the displacement sensor temperature and/or an environmental temperature thereof is measured in order to obtain a measured temperature. A measured sensor value is determined which applies at such measured temperature. The method further involves determining a temperature coefficient, which is at first unknown, by using a first function and a second function, the first function being based on the temperature-dependency of the measured sensor value, and the second function being based on the known dependency of the sensor temperature coefficient on the measured sensor value at a reference temperature. This dependency relates to a sensor model, and the determination of the temperature coefficient includes solving the two functions using iteration. The measured sensor value is then temperature-compensated to form a temperature-compensated measured sensor value by converting the measured sensor value into the measured sensor value applicable at the reference temperature using the temperature coefficient applicable to the sensor and the temperature difference between the measured temperature and the reference temperature. Such temperature-compensated measured sensor value is converted into a measured value with a mechanical length unit through calibration.
The invention provides the advantage that no hardware devices are required for its implementation, which would increase cost, with the exception of a memory expansion which might perhaps be necessary at marginal cost, since the sensor evaluation method is carried out by a program.
A further development of the invention provides the advantage that the calibration according to the invention need only be carried out once, i.e., at the initial utilization of a (new) vehicle clutch.
Another further development of the invention makes it possible to adjust the state of the fully engaged clutch. The previously known method cannot be used for a clutch, and can only be used with TARGET positions that can be permanently predetermined.
Another further development of the invention provides the advantage that a warning can be issued to the driver in the course of vehicle operation when the clutch lining no longer has a given remaining lining thickness. In such manner, the embodiment of the invention provides the concomitant benefit that a clutch can be replaced in a timely manner, before destruction of said clutch, while a premature replacement of a clutch is avoided when it could still further perform, for example, for another 100,000 km of vehicle operation.
Another further development provides the advantage that iterative calculation methods which are used are carried out at non-critical times, and the result is stored so as to be available at critical times without delay and without stress on the computer.
Another further development provides the advantage that the recognition of the state of the sufficiently disengaged clutch can be ascertained very easily and independently from the state of wear, in order to shift through a partially or fully automatic transmission gearshift.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings in which like reference numerals designate the same elements.